1. Field of Invention
The present invention relates to a resonant tag equipped with a resonant frequency circuit which resonates with a radio wave transmitted at a particular frequency from a detector, and also relates to processes for producing resonant tag and for fabricating a circuit-like metallic foil sheet for a resonant tag and the like.
2. Description of Prior Art
As is well known, resonant tag Is a tag equipped with a resonant circuit which resonates with a radio wave transmitted at a particular frequency. Thus, when the resonant tag is brought into an area in which the radio wave of the particular frequency is transmitted, the resonant tag readily reacts to operate a buzzer or an alert lamp of a detector. Thus, to avoid shoplifting, the resonant tags are attached to goods and the like in department stores, discount stores, rental shops for video tapes, compact disk (CD) shops, etc. When the payment is settled at the cashier, the resonant tag is removed from the goods or the resonant circuit of the resonant tag is destroyed. However, if a shop's item is passed through a gate equipped with a particular detector without finishing the payment, the resonant tag attached to the item activates a buzzer, a lamp, etc., to give off an alert signal.
Basically, a resonant tag is composed of a plastic film (as a dielectric), having a spiral (or a coil-shaped) circuit (hereinafter referred to as a "coil circuit") on at least one side of the film, and a circuit for use as an electrode plate of a capacitor or another coil circuit which functions also as a capacitor on the other side of the film. If necessary, the resonant tag is laminated on a sheet of a base material such as a paper.
A typical resonant tag is illustrated in FIG. 6. Referring to FIG. 6, a resonant tag comprises a plastic film 22 which functions as a dielectric, a coil circuit 23 provided with a metallic foil, and a metallic foil circuit 24 for a capacitor electrode plate. The coil circuit 23 comprises a capacitor electrode plate section 23A on one end thereof and a circuit terminal section 23B on the other end thereof. The metallic foil circuit 24 for the capacitor electrode plate also comprises a capacitor electrode plate section 24A on one end thereof and a circuit terminal section 24B on the other end thereof corresponding to the coil circuit 23. The circuit terminal sections 23B and 24B, which are formed by sandwitching the plastic film 22, are connected with each other to form a resonant circuit by destroying the layer of the plastic film 22 using a mechanical means such as pressing. A complete resonant tag is obtained in this manner.
In addition to a resonant tag above, there is also proposed a resonant tag having no capacitor electrode section formed on the end portion of the coil circuit. In the proposed structure, coil circuits are formed on both sides of a plastic film in correspondence with each other to utilize the circuit itself as the capacitor electrode plate.
A resonant circuit is composed of a resistance R, an inductance L, and a capacitance (capacity of capacitor) C. The capacitance C is formed by providing a metallic foil such as a coil circuit, on both sides of a plastic (resin) film as a dielectric, and the resistance R is provided by the metallic foil constituting the circuit. Thus, to obtain a resonant tag with a predetermined resonant frequency, the constitution of the materials should be determined as to enable a circuit with strict accuracy in dimension and tolerance.
In the light of the aforementioned circumstances, the coil circuits have been formed conventionally by using, as the base material, a plastic film (a dielectric) laminated on either or both sides thereof a metallic foil such as an aluminum foil. In the same manner as that for fabricating a printed circuit board, the metallic foil on the plastic film are patterned into the circuit by first printing the predetermined pattern using an etching-resistant ink and then etching the printed metallic foil using a chemical such as an acidic or an alkaline solution. Otherwise, the coil circuits are formed by means of photoresist etching.
However, the etching process using a chemical not only consumes time in dissolving the metallic foil, but also has problems yet to solve concerning the post treatment of the waste etching solution.
Concerning the plastic films for the dielectric, extruded films of polyethylene and the like have been used conventionally. Because extruded plastic films are under various constraints attributed to the production process subject to various conditions such as the slit width of the molding die and the extrusion pressure applied to the resin, films with limited thickness and tolerance in thickness can be obtained. More specifically, the plastic films obtained to present by extrusion molding are problematic in that a film below a certain thickness cannot be obtained, i.e., there is limitation in lower-limit under molding, and that the tolerance in thickness cannot be reduced beyond a certain limit. A plastic film thus obtained is laminated with a metallic foil by either thermal pressing the extruded film while it is in a semimolten state with a metallic foil, or adhering the extruded film with a metallic foil using an adhesive. At any rate, both methods cannot yield stably a laminate comprising a dielectric layer of uniform thickness. Furthermore, because of the inherently strong covering power of the molded plastic film, the connection between the circuits on the surface and the back of the plastic film cannot be readily formed, or the resonant circuit once established cannot be easily destroyed, i.e., deactivation of resonant frequency characteristics is hard. These are other problems in using the conventional extruded plastic films.
Prior art processes for fabricating printed circuit boards propose stamping out a thick metallic foil into a predetermined circuit pattern and adhering the resulting circuit to a substrate. This process, however, is not suitable for resonant tags and the like because a flexible board cannot be obtained from a thick metallic foil. A process for fabricating a flexible circuit proposes die stamping, which comprises adhering a metallic foil to a substrate and stamping out the metallic foil into a predetermined circuit pattern, and peeling off the metallic foil from the unnecessary portions. However, this process is disadvantageous in that it excludes the use of a metallic foil not strong enough for the peeling off, and that the peeling efficiency is greatly impaired if unnecessary portion is discontinuous.